Review of Poly(bis(dialkylamino)phenylene vinylene)s as Corrosion

Chapter 18

Review of Poly(bis(dialkylamino)phenylene vinylene)s as Corrosion-Inhibiting Materials 1

Peter Zarras, John D. Stenger-Smith , Gregory S. Ostrom, and Melvin H . Miles

Downloaded by UNIV OF BATH on July 1, 2016 | http://pubs.acs.org Publication Date: September 16, 1999 | doi: 10.1021/bk-1999-0735.ch018

Research and Technology Group, Naval A i r Warfare Center Weapons Division, China Lake, C A 93555-6100

A brief history of research on conducting polymers, with specific emphasis on results in the area of conducting polymers as corrosionprotective coatings is presented. The synthesis and corrosion inhibiting properties of poly(bis(dialkylamino)phenylene vinylenes at isotonic sea water conditions are presented and discussed.

1

On a historical note, polyaniline was first made as far back as 1862 by Letheby . K n o w n as "aniUne black", this material was formed by oxidation of aniline und^r mild conditions ' . Aniline black was an important material for dyeing and printing . Conducting polymer research has roots back to the 1960s when Pohl, Katon, and others first synthesized and characterized semiconducting polymers " and conjugated polymers . The discovery of tfie high conductivity of poly(sulfurnitride^ / ^ N ) , a polymeric inorganic explosive , and its interesting electrical properties " was a step towards conducting polymers as they are known today. The beginning of conducting polymer research began nearly a quarter o f a century ago, when films of poly acetylene were found to show tremendous increases in electrical conductivity when exposed to iodine vapor ' . This was the first report of polymers with high-electrical conductivity. The procedure for synthesizing polyacetylene was based upon a route discovered in 1974 by Shirikawa through serendipitous addition of a thousand times the normal amount of catalyst during aie polymerization of acetylene . Over the past two decades, there have been several excellent reviews on conducting polymers " . Conducting polymer research is evolving rapidly enough that yearly reviews are almost a necessity. Today, there are hundreds of articles on conducting polymer research published every year. The conducting forms of conducting polymers are usually classified as the cation salts of highly conjugated polymers. The cation salts are obtained by electrochemical oxidation and electrochemical polymerization or chemical oxidation (removal of an electron). It is also possible to obtain the anion salts of the same highly conjugated polymers, which are also conducting but much less stable than the cation counterparts, by either electrochemical reduction or by treatment with reagents such as solutions of sodium naphthalide ' . In general, a conjugated backbone and/or a backbone that has a low enough oxidation potential is necessary but not sufficient for the electrically conducting form of a polymer to remain stable in the x

1

280

Corresponding author.

U.S. government work. Published 1999 American Chemical Society

Hsieh and Wei; Semiconducting Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

281

presence o f air and water or end-use conditions such as inside an automobile or home. Figure 1 lists some examples of conducting polymers. Polyaniline, one of the most studied conducting polymers (in general as well as for corrosion protection), is usually obtained by protonation o f what is called the emeraldine base form, shown i n Figure 2. The protonation reaction does not change the number o f electrons in the polymer backbone. However, starting at die leucoemeraldine form of polyaniline, one would obtain the emeraldine salt (conducting) form of polyaniline by an oxidation reaction. Protonic doping has also been observed i n the case of alkoxy substituted poly(para-phenylene vinylene) (PPV) ' . 3 8

3


Corrosion Protection In the area o f potential applications, conducting polymers are used as replacements for metals because the conducting polymers have potentially unique and or superior properties, or because the metals are toxic or damage the environment. Current methods of corrosion protection (particularly marine coatings) do not last very long and are coming under increased scrutiny by the Environmental Protection Agency ( E P A ) . F o r example, the use o f chromium (especially hexavalent chromium) and cadmium for anti-corrosion coatings will soon be banned. A mechanism for corrosion protection involves the use of a sacrificial electrode, such as a zinc coating, which w i l l corrode (oxidize) in the place o f the substrate. However, the coatings do not last very long. The oxidized zinc metal is dissolved by water or moisture. F o r this reason there are extreme environmental concerns since toxic metals are being released. Barrier coatings such as epoxy are employed extensively but are not very durable/robust once a pit or hole in the coating has been formed. The corrosive species then attacks the underlying metal and, thereby, increases the exposed surface, accelerating the corrosion process. The corrosion.inhibiting properties o f conducting polymers were suggested by MacDiarmid in 1985. Initial studies on the protection o f metal surfaces against corrosion by conducting polymers was reported i n the hterature that same y e g r M u c h o f the work on corrosion protection has focused on Dolyaniline ( P A N I ) " , but also has been extended to other conjugated polymers " . A major type o f corrosion occurs by oxidation of a metallic surface by a saltwater medium to produce oxides and hydroxides. A s these form, soluble species are produced, the surface pits increase the surface area, and the rate of decomposition accelerates. One way to provide corrosion protection is to coat the metal with a barrier to prevent the reactive species from reaching the surface. Galvanization with zinc (or other metal with low enough oxidation potential) prevents corrosion via the creation o f an interfacial potential at the metahzinc interface. The zinc will corrode preferentially. While the reactive species may come in contact with the metal, the increased oxidation potential causes the metal to be unreactive. Prior work utilizing P A N I as a corrosion-protection coating shows that it works quite well. In fact, exposed metal surfaces adjacent to conducting polymer coatings (scratches or edges) are unreactive to corrosion as reported by Thompson and coworkers ' . The corrosion protection properties o f P A N I on aluminum i n acidic media have also been studied . Therefore, corrosion protection is an area where conducting polymers have great potential. If superior performance o f conducting polymers can be demonstrated under maritime conditions (which vary greatly depending upon use and environment), there is a potential for multi-billion dollar savings. For example, estimates from the United States Navy indicate that corrosion costs o f aircraft (presented at the Third International Conference on Aircraft 46


282

Poly(acetylene) (PA)

Poly(pyrrole) (PPy)

H


PolyCpara-phenylene vinylene) (PPV)

Poly(para-phenylene) (PPP)

«Ο* Poly(thiophene) (PT)

Poly(3,4-ethylenedioxythiophene) (PEDOT)

0

\)

Figure 1. Structures of Conjugated Polymers i n Their Respective Neutral Forms.

0-K>*-0*0" Emeraldine Base

x-

x-

Emeraldine Salt (Conducting)

x-

x-

Emeraldine Salt (Conducting) Figure 2. Structures of Polyaniline.



283

Corrosion, August 25-27, Solomons Island M D ) are several billions g|f dollars per year and corrosion costs of ships are tens of billions of dollars per year . There are several proposed mechanisms for corrosion protection, one or more of which could be occurring at any time. The first is a simple galvanic process by which the polymer has a lower oxidation potential than the metal it is protecting; the polymer is preferentially oxidized. Because oxidized polymers are usually insoluble and, therefore, do not dissolve away as zinc does, corrosion protection with conducting polymers should last longer. Another proposed mechanism is that the polymer reacts with the surface of the metal, requiring that the polymer have an oxidation potential higher than that of the mptal. The surface of the metal reacts with the polymer and forms a passivating layer which inhibits further corrosion by either setting up a barrier or by changing the surface potential or both. One possible disadvantage to using polyaniline is that the corrosion-protection ability is p H dependent. In acidic media, polyaniline-coated mild steel coupons corrode a hundred times slower than counterparts, while in p H 7 media, the P A N I coated material corrodes twice as slowly . Because the p H of seawater is around 8.0 to 9.4 depending upon season and location, it is unclear or unproven that P A N I w i l l provide any additional corrosion protection for ocean-going vessels. This could be explained by the p H dependence of the structure of P A N I . A t low p H , the conducting emeraldine salt is the predominant form; at high p H , the non-conducting emeraldine base is the predominant form. It appears that the conducting form is required for the formation of the passivation layer. In summary, the amount of corrosion protection is controlled by the type of polyaniline (emeraldine base versus emeraldine salt) and the characteristics of the corrosion environment (acidic medium, aqueous sodium chloride, or seawater) and also by adhesion to the substrate. Studies on the marine application of corrosion protection capability of conducting polymers w i l l need to be performed in solutions isotonic with seawater and/or a salt fog according to American Society for Testing of Materials ( A S T M ) methods . F o r corrosion protection, it may be necessary to develop conducting polymers that do not have the p H dependence of conductivity that P A N I has or to formulate P A N I in such a way that it provides corrosion protection at basic p H . There may be some concern that all conducting polymers behave as polyaniline, that is, that all conducting polymers exhibit the same p H dependence of corrosion protection that polyaniline does. However, since polyaniline is made conducting by treatment with acid whereas most other conducting polymers are not, it is extremely unlikely that all conducting polymers w i l l exhibit similar p H dependence of corrosion inhibition. Synthesis vinylene)

of PoIy(2,5-bis(n-methyl-n-propyl)amino (BAMPPV)

phenylene

The addition of bis(dialkyl amino) substituents onto the P P V backbone is of interest for several reasons. First, the amino groups are generally stronger electron donors than alkoxy groups (provided that the resulting amino substituted polymer structure is planar) and should bring the oxidation potential of the polymer down around 0 volts vs. S C E , making the conducting polymer even more stable. A l s o , amine functionalized polymer should adhere fairly well to aluminum. Finally, amino groups can also be quarternized, which could be exploited to make the polymer water soluble. The synthesis of amino functional P P V presented some serious synthetic challenges . Usually, a radical halogenation step is used i n making the precursors to P P V . For example, 1,4-dimethyl benzene would be chlorinated or brominated to make a precursor to P P V . Although this method can be adapted well for the alkoxy


284

derivatives, it cannot be used for amino derivatives because the reaction is dangerously exothermic. The danger of this type of reaction was established back in 1957, when a fatal accident occurred . Another method, used generally for dialkoxy derivatives, is to chloromethylate the dialkoxy substituted compounds. This reaction w i l l not work with amines because the acidic conditions used w i l l protonate the arnine making it unreactive. Therefore, another synthetic strategy was developed. This method, shown i n Scheme 1, does not involve any of the problem steps mentioned above.

H NC H


2

3

Br

2

7

CHCI3

Reflux

N-C3H7 H

C H 3

7

v;

H C - -N

H C

3

CH3I THF/NaH

3

X

DIBAL

0

CH

N-C H 3

3

7

HC 3

•HQ /C H H C-N 3

7

3

SOCl

Dry HC1

N-C H 3

HC

HC

3

3

9

7

·ΗΟ

6 •HC1

C3H7

H C-N 3

H C—Ν 3

CI

KOBu THF

N-C H 3

HC 3

7

-HC1

7 Scheme 1. This method a l l o c s the synthesis of fairly pure polymer and is being improved to allow for scaleup . The electrical and electrochemical properties of this polymer are currently being studied. The neutral form of the polymer is orange-red in color and


285

the absorption maximum is 460 nanometers. This absorption maximum is much higher i n energy than expected, so it is possible that the polymer backbone is nonplanar.


C o r r o s i o n Studies of B A M P P V B A M P P V was coated onto Type II anodized aluminum T3 plates (5.1 χ 5.1 χ 0.15 cm, A = 55 c m ) in saltwater for a 12 month immersion study. Constant current (galvanostatic and constant potential (potentiostatic) methods were used to investigate the corrosion of these aluminum plates. The electrochemical studies were conducted in concentrated saltwater solutions by dissolving 70.13 grams of sea salt (Bio-Sea Marine M i x ) in 1200 milliliters of deionized water. This produced a solution of approximately 1 molar i n N a C l with a specific gravity of 1.04257, with a p H o f approximately 8.1, which contained the trace elements of seawater. Potentiostatic and galvanostatic studies of an (unanodized) aluminum plate (immersed for 1 month) in this salt solution are shown in Figure 3. The two methods give approximately the same results, showing the pitting potential (the potential at which the current increases rapidly due to corrosion) near -0.6 volt vs. A g / A g C l . The electrochemical behavior is very different for anodized-aluminum plates (immersed for 1 month), as shown in Figure 4. There is no measurable current for this potentiostatic study to within 0.001 milhampere from -0.600 to -0.300 volts. The pitting potential is shifted markedly from that shown in Figure 3 to about -0.28 volts vs. A g / A g C l . Despite the anodic potentials applied in Figure 4, currents larger than 15 miUiamperes are not observed. Further increases in potentials out to 3.00 volts were investigated; the largest current obtained was only 25 milliamperes. Detailed examination of the aluminum plate indicated that a few isolated regions on the edges o f the plate were the main contributors to the anodic current. During the time period of this potentiostatic study, the anodic current gave a yield of 1.380 coulombs. A l s o shown i n Figure 4 are exactly the same potentiostatic measurements for an anodized aluminum plate coated with B A M P P V (immersed for 1 month). The striking feature is that very little corrosion current is observed. There is no measurable current to within 0.001 milliampere from -0.600 to +0.45 volts. The corrosion potential, i f there is one, is near 0.525 volts vs. A g / A g C l . This increase in overvoltage corresponds to an activation energy barrier increase o f 57.9 kilojoules/mole. The current increases only slightly to 0.071 milliampere at 0.80 volts and then decreases. There is no further increase in the current even out to 3.00 volts. The coulombic measurements during this potentiostatic study yielded only 0.00358 coulombs. Therefore, based upon the coulombic measurements, the polymer-coated anodized-aluminum plate yielded only 0.26% of the corrosion measured for the uncoated anodized-aluminum plate. This is quantitative evidence that polymer coatings can substantially reduce pitting corrosion of aluminum. Figure 4 also shows a potentiostatic study after the plates were immersed i n isotonic sea water for an additional 5 months (6 months total). The pitting potential is still much higher for the coated film (0.225 volts vs. A g / A g C l ) , which is about 75% of the initial value, giving an activation energy barrier for corrosion of 43.4 kilojoules/mole. In addition to the studies shown in Figures 4 and 5, several long-term (days) constant-current electrolysis experiments were conducted with polymer-coated and uncoated-aluminum plates. In each long-term study, the corrosion pits that developed were always significantly less for the polymer-coated plates. Further studies of the B A M P P V plates extending to a 12 month study were accomplished using the same methods for the 6 month study. Figure 5 shows similar behavior for the anodized-aluminum plates as in Figure 4. There was no measurable current for this 12 month potentiostatic study to within 0.0001 milliampere from -



286

1000 • Potentiostatic study

< ε

ιοο

i

* Galvanostatic study

c ο

3 Ο

-0.8

-0.6

Potential/V

-0.4 vs.

-0.2

Ag/AgCl

Figure 3. Potentiostatic and Galvanostatic Electrochemical Studies of an Unanodized Aluminum Plate in the Sea Salt Solution.


287


1000

Potential/V vs.Ag/AgCI

— • —

Open squares-Uncoated plate (6 months)

—Ο—

Open circles-Coated plates(6 months)

.... _... a

Half squares-Uncoated plate (initial)

— Δ —

Open triangles-Coated plate(initial)

Figure 4. Potentiostatic Electrochemical Studies of Anodized-Aluminum Plates (1 and 6 month).


288

Potentiostatic electrochemical studies of anodized aluminum plates (pH=-8.1) -3


1000

-

1

1

0

2

3

Potential (V) vs Ag/AgCl

• * • •

Uncoated plate (12 month) Coated plate (12 month) Uncoated plate (10 month) Coated plate (10 month)

Figure 5. Potentiostatic Electrochemical Studies of Anodized Aluminum Plates (10 and 12 months).


289


0.600 to -0.300 volts and detailed examination of the aluminum plate still indicated that only a few isolated regions of the plate due to edge effects were the main contributors to the anodic current. Furthermore, the pitting potential is still much higher for the coated film (around 0.2 volts vs. A g / A g C l ) , giving an activation energy of around 38 kilojoules/mole, which is 65% of the initial value. The results from the 10 month study show some very small current spikes, which could be due to edge effects or instrument artifacts. These spikes make quantitative interpretation of the 10 month data difficult, but generally, the 10 month results are consistent with the overall study. Scanning electron microscopy ( S E M ) was also performed on anodized aluininurn plates and B A M P P V - c o a t e d aluminum plates (both immersed for 6 months). The results from the S E M , shown i n Figure 6, show that the polymer coating provides a significant degree of corrosion protection.

Figure 6. (a) Β A M P P V - C o a t e d Aluminum with M i n i m a l Corrosion and (b) Uncoated Aluminum with Severe Corrosion.


290

Summary and Dicussion A new conducting polymer, poly(2,5-bis(N-methyl-N-propyl)amino phenylene vinylene), was synthesized and characterized. This conducting polymer is a very promising candidate for corrosion protection of aluminum i n isotonic sea water. The time is ripe to expand research i n this area, with specific focus on shipboard applications and neutral to slightly basic p H , and incorporation o f polyaniline for use in shipboard acidic environments. This conducting polymer program should have some focus on corrosion protection at neutral to basic p H , include new materials synthesis to meet these needs, and formulations of polyaniline that might inhibit corrosion at higher p H .


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Review of Poly(bis(dialkylamino)phenylene vinylene)s as Corrosion

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